ATMOSPHERIC COMPOUND
IS DOUBLE-EDGED SWORD IN CLIMATE CHANGE

SAN FRANCISCO – Recent studies suggest that an atmospheric compound
derived primarily from coal combustion may have contradictory effects
on the earth's climate.

Under many conditions, sulfuric acid may cool the earth’s atmosphere.
Sulfuric
acid particles seem to scatter ultraviolet light back into space before
it has a change to enter the troposphere – the bottom layer of earth’s
atmosphere. But if conditions are right, this same chemical can warm the
earth by combining with other compounds in the atmosphere to form clouds.

Heather
Allen

Researchers at Ohio State University looked at the interaction of sulfuric
acid and methanol
and what the compounds' combined effect might mean to global climate change.
Both compounds are usually found in aerosol form in the upper atmosphere.

Scientists believe that methanol comes primarily from natural sources,
such as oceans, forests and the decay of organic matter. While there are
a few natural sources for sulfuric acid, such as volcanoes and marine
sea spray, its precursor – sulfur dioxide – comes mainly from
the burning of coal. In the atmosphere, sulfur dioxide is oxidized primarily
by atmospheric moisture, resulting in sulfuric acid.

The
researchers found that sulfuric acid combines with a small amount
of methanol– essentially starting points for cloud formation.
But the conditions must be right in order to create methyl sulfate.
The interaction between sulfuric acid and methanol affects global
climate change and the aerosol picture.

Sulfuric acid molecules in atmospheric aerosols can act as sort of a
force field by reflecting light and heat back into space, said Heather
Allen, a study co-author and an assistant professor of chemistry
at Ohio State. This reflection contributes to a cooling effect on
the earth. Methanol by itself doesn't really have an effect on climate
change.

But when the two molecules get together – about 5 to 10 percent
of the methanol in the atmospheric aerosols reacts with sulfuric acid
– they form methyl sulfate. Methyl sulfate is less volatile than
methanol, meaning there's less chance that methyl sulfate will evaporate
or be vaporized.

And while it seems like a relatively small amount of methanol gets converted
to methyl sulfate, it's still enough to have an impact on global climate
change, Allen said.

She and colleague Lisa Van Loon, a doctoral student in chemistry at Ohio
State, found that methyl sulfate's stability provides a springboard for
cloud formation – water droplets collect on the stable molecules
and eventually form clouds. Instead of causing light and heat to bounce
back into space, most clouds create a warming effect by trapping light
and heat in the atmosphere.

The researchers used a laboratory technique called Raman spectroscopy
to analyze the behavior of methanol, sulfuric acid and methyl sulfate.
They focused a beam of laser light onto a sample of each substance in
order to analyze differences in the bonds that hold the molecules together.
The frequencies of the resulting wavelengths told the researchers how
the compounds behaved, and also how methanol and sulfuric acid interacted.
From this information, they could determine what each compound might do
in the atmosphere.

The researchers found that sulfuric acid combines with a small amount
of methanol– essentially starting points for cloud formation.

But the conditions must be right in order to create methyl sulfate, Allen
said.

"The atmospheric chemistry community is trying to understand what
conditions let these atmospheric particles combine, or cause them to stay
aerosol-sized," Allen said, adding that cloud particles are about
three times the size of aerosol particles.

"Right now these aerosols are probably helping to slow down the
human-induced warming effect on the earth, but it's a complicated balance
that we're struggling to fully understand," she said. "We certainly
know that the earth is warming at a rate that isn't totally natural. It's
the warming rate that we're more concerned about.

"More aerosols emitted into the atmosphere may lead to cooling,"
Allen continued. "But if these aerosols are able to combine with
other compounds and ultimately form clouds, it could have a warming effect.
There's a complex balance between warming and cooling."